The present invention relates to a precompensator to correct the dispersive effects of optical fiber. More specifically the present invention relates to a terahertz electromagnetic radiation emission and detection system that utilizes a precompensator, optical fiber, and pulsed laser.
In the present invention extremely short optical pulses in the femtosecond range generated by a laser are transferred from a laser to a terahertz generator by way of an optical fiber cable. The terahertz generator is comprised of a material that, when illuminated with a short optical pulse, generates electromagnetic radiation in the terahertz range (10 GHz to 50 THz). These materials fall principally into two large categories, photoconductive terahertz generators and non-linear optical generators. In the former category the incident photons generate electrical carriers, both holes and electrons, which are then accelerated by a voltage potential within the material that is either externally applied or internally present due surface potentials in semiconductors. This charge motion in turn generates an electromagnetic field that normally consists of a single or half-cycle of radiation in the terahertz range. The second category of THz generators consists of materials that utilize non-linear optical methods to generate THz radiation. These materials have a non-linear susceptibility, .chi..sup.(2), .chi..sup.(3), or .chi..sup.(4) that causes the input optical pulse to generate a polarization state due to the equation: EQU P.sub.NL =.chi..sup.(i) (E).sup.i
Where P.sub.NL is the non-linear polarization state of the material, and E is the electrical field of the incident optical pulse. This method is known by a number of names to describe the various physical processes taking place. Some of the effects known to occur are, the inverse Franz-Keldysh effect, electric-field-induced optical rectification, the Stark effect, and Cherenkov radiation. This effect will heretofore be referred to as optical rectification since the scientific literature generally accepts this term to encompass all of the effects.
In order to successfully deliver high contrast, sub-100 femtosecond pulses from a laser, one must effectively control the dispersion in an optical fiber through the use of a precompensation device. Dispersion is the spreading and/or distortion of a light pulse as it travels down the length of an optical fiber. Different wavelengths or colors of light travel at different velocities through a fiber, which tends to widen an optical pulse. This phenomena result from the non-linear frequency dependence of the refractive index of silica used in optical fibers. For a more detailed description with respect to optical dispersion see Steven John Kane, "High-Order-Dispersion Control for the Amplification and Compression of Femtosecond Laser Pulses" (1997) (Ph.D. dissertation, University of Michigan (Ann Arbor)) and James VanHartness Rudd, "Advanced Techniques for the Amplification of Sub-100-femtosecond Pulses in Ti:Sapphire-Based Laser Systems" (1996) (Ph.D. dissertation, University of Michigan (Ann Arbor)).
The dispersion of light leads to a potential corruption of the terahertz signal. The problem can be corrected by precompensating the signal for any stretching caused by the material dispersion characteristics of a fiber optic cable. An optical pulse may be given dispersion characteristics that are equal and opposite to the dispersion generated by an optical fiber, allowing the exact reconstruction of a pulse as it exits the optical fiber.
The present invention is concerned with the generation of terahertz electromagnetic radiation by a pulsed laser in a commercially packaged system. In previous applications such as in a lab environment a laser can be pointed directly through space at an optical switching element with negligible dispersive effects. To allow the commercial use of such a system the present invention must be industrially hardened and packaged. A laser pulse in a room environment may be deflected by objects or people and will suffer degradation from atmospheric effects, unacceptable conditions in an industrial environment. By incorporating an optical fiber cable into the present invention, the laser light is given a predetermined path of travel and allows the present invention to be precisely aligned, ruggedly seated, and bundled into compact form. Given the need for an optical fiber cable to package the system the problem of dispersion now exists, necessitating a precompensation device to maintain the fidelity of the optical pulses traveling through the optical fiber cable.
Further objects, features and advantages of the invention will become apparent from a consideration of the following description and the appended claims when taken in connection with the accompanying drawings.